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292

PRACTICAL ORGANIC CHEMISTRY

PREPARATION 74.
Potassium Benzenestdphonate.The formation of sul-
phonic acids by the action of sulphuric, acid, &c., on the aromatic
hydrocarbon is a special property of aromatic hydrocarbons,
although, in a few cases, paraffins have been found to react in a
similar manner. The process is called " sulphonation." In
place of cone, sulphuric acid, fuming sulphuric acid, i.e., an acid
containing varying proportions of sulphur trioxide (see Prep. 109,
p. 226), and, occasionally, chlorosulphonic acid, C1SO2OH, are
used. In the two latter cases sulphones are sometimes formed
as a by-product,
2C6H6 + SO, = (C(;H5),SO., + H,0.
2QH + C1S03OH = (C(jlIB)2SOa + HC1 4- H..O.
The sulphonic acids are also obtained by the oxidation of
thiophenols, a reaction which, at the same time, indicates their
constitution,
QH8SH + O., = C6H5SO,II.
TTie majority of aromatic sulphonic acids are very soluble in
water, and are difficult to obtain in the crystalline form. On
the other hand, the sodium or potassium salts generally crystal-
lise well, and it is customary to prepare them by pouring the
sulphonic acid directly after sulphonation into a strong solution
of sodium or potassium chloride (Gattermann).
The sulphonic acids decompose on heating into the hydro-
carbon and SO3. This reaction is greatly facilitated by heating
with cone, hydrochloric acid to 150180 (Jacobsen), or by
passing superheated steam into the sulphonic acid mixed
with moderately strong sulphuric acid (Armstrong).
This method is sometimes used for separating hydrocarbons,
one of which is more easily sulphonated than another. The
sulphonic acid is separated from the unchanged hydrocarbon,
and the hydrocarbon is then regenerated from the sulphonic
acid.
The salts of the sulphonic acids undergo the following re-
actions :
T. By fusion with caustic alkalis, phenols are prepared (see
Preps. 106 and 219),
C6H,SO,Na + NaOH = CHaOH + Na2SO3.